A touch panel is a device for inputting, with a touch (pressing) of a finger, a stylus, etc., information to an information processing device such as a computer in an interactive manner.
Touch panels are classified by their principle of operation into different types, such as resistive, capacitive, infrared, ultrasonic wave, and electromagnetic induction coupling. Resistive touch panels and capacitive touch panels can be incorporated into display devices, etc., at low cost and are thus widely used in recent years.
A resistive touch panel, which is composed of, for example, a pair of glass substrates disposed to face each other, transparent conductive films provided as resistive films on the entire inner surfaces of the pair of glass substrates, an insulating spacer sandwiched between the pair of glass substrates and forming an airspace between the transparent conductive films, and a touch location detection circuit for detecting touched locations, is placed on the front surface of the display screen of a liquid crystal display panel, for example.
In the resistive touch panel thus configured, with a touch of the front surface of the display screen, the transparent conductive films are brought into contact with each other (making a short circuit) to cause a current to pass between the pair of transparent conductive films. And in this touch panel, the touch location detection circuit detects the touched location in accordance with a variation in voltage occurring when the current passes between the pair of transparent conductive films.
However, the resistive touch panel, in which the pair of transparent conductive films are disposed to face each other with the airspace interposed therebetween, has a drawback in that the airspace causes difference in refractive index to increase, resulting in a reduction in light transmittance.
A capacitive touch panel has the following configuration.
FIG. 12 is a schematic cross-sectional view of a liquid crystal display device 150 including a typical capacitive touch panel.
The liquid crystal display device 150 is composed of: a liquid crystal display panel 100 including an active matrix substrate 110, a counter substrate 120 disposed so as to face the active matrix substrate, and a liquid crystal layer 130 provided between the active matrix substrate 110 and the counter substrate 120; a back light 105 placed under the liquid crystal display panel 100 with a polarizing plate 101 and a diffusion sheet 103 interposed therebetween; and a touch panel 140 placed over the liquid crystal display panel 100 with a polarizing plate 102 interposed therebetween. The touch panel 140 is fixed on the display screen which is the upper side of the liquid crystal display panel 100 by an adhesive layer 104, such as double-faced adhesive tape.
The touch panel 140 includes a glass substrate 141, a location detection transparent electrode 142 provided on the entire surface of the glass substrate 141, location detection electrodes (not shown) disposed on the periphery of the location detection transparent electrode 142 at regular pitches, and a location detection circuit (not shown) for detecting touched locations.
In this touch panel 140, a touch of the front surface of the display screen, i.e., the surface of the glass substrate 141, causes the location detection transparent electrode 142 to be grounded at the touched point through the capacitance of the human body, resulting in a variation in resistance value between each location detection electrode and the grounded point. And in this touch panel 140, the touch location detection circuit detects the touched location in accordance with the variation in resistance value between each location detection electrode and the grounded point.
In this liquid crystal display device 150, the number of glass substrates (111, 121, and 141) is three, which is one less than that in a liquid crystal display device including the above-described resistive touch panel, while the airspace between the pair of transparent conductive films existing in the resistive-touch-panel-equipped liquid crystal display device is not present. The liquid crystal display device 150 thus has excellent light transmittance.
Also, Patent Document 1, for example, discloses a capacitive touch panel that has a configuration in which a first transparent substrate provided with a transparent conductive film for touch location detection is bonded on the touch side thereof to a second transparent substrate for glare prevention by a transparent adhesive. Patent Document 1 describes that this configuration prevents damage to the transparent conductive film while allowing an increase in productivity.
However, a display device which uses the above-described touch panel placed on the front surface of the display screen of the display panel has a problem in that the touch panel itself increases the thickness and weight of the entire device or the cost.
It is therefore known that, in order to reduce the thickness and weight of the device, components included in the display device are also used as the glass substrate and the location detection transparent electrode forming the touch panel, thereby eliminating the need for forming the glass substrate and the location detection transparent electrode.
For example, Patent Document 2 describes a touch sensor and display device in one, in which the display device, which includes an active matrix substrate having a plurality of pixel electrodes arranged in a matrix, and a transparent common electrode that faces the active matrix substrate, is provided with: a liquid crystal display circuit for supplying voltage or current for display to the transparent common electrode; a location detection circuit for detecting currents flowing from a plurality of points in the transparent common electrode; and a switching circuit for electrically connecting one of these circuits with the transparent common electrode.
Also, Patent Document 3 describes a capacitive touch panel, in which a common transparent electrode, a liquid crystal, and a display transparent electrode are sequentially stacked between two transparent insulating plates, current detectors for detecting current flowing between a contact object, such as a finger, and the common transparent electrode through the transparent insulating plate that is disposed close to the common transparent electrode, that displays thereon characters or an image, and that is touched by the contact object, are mounted on the four corners of the common transparent electrode in order to detect the coordinates of the location of the touched part on the transparent insulating plate, and a signal processing circuit is included for calculating the coordinates of the location of the touched part from current signals from the current detectors at the four corners which are affected by a variation in capacitance occurring due to the touch of the contact object on the touched part on the transparent insulating plate.    Patent Document 1: Japanese Laid-Open Publication No. 5-324203    Patent Document 2: Japanese Laid-Open Publication No. 2003-66417    Patent Document 3: Japanese Laid-Open Publication No. 2003-99192